my brothers stuff

My brother was going on a week long trip when he rang in a panic asking for me run down and give him his train ticket. He also asked for me to through a bag under his bed. It had an empty box off lube and a used condom. I felt sick knowing he was doing it when I was right upstairs.I wasn't suppose to look in it. But now I'm not sure if I can even look at him. I feel like I should tell my mum but he'd hate me and ruin our trust